In the case of the imaging examination of objects, (e.g., patients), by Magnet Resonance Imaging (MRI), a (e.g., permanently installed) body coil, which may also be called a whole body resonator, is currently used for exciting magnetic resonance signals, and this has an antenna unit for this purpose. The magnetic resonance signals may have frequencies around a Larmor frequency. While it is possible to also use the body coil for receiving the magnetic resonance signals, dedicated local coils may be used for this purpose, however. These are conventionally positioned close to the patient and consequently supply a high Signal-to-Noise Ratio (SNR). Furthermore, modern local coils conventionally include a large number of individual coil elements. Magnetic resonance signals of a coil element received during a receiving interval may be amplified by a receive amplifier. The high number of coil elements means imaging techniques, in particular parallel imaging techniques, such as, for example, Sensitivity Encoding (SENSE) or Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) may be applied, which in part lead to significantly reduced scanning times with the same image quality.
To achieve a high image quality, it is desirable to reduce coupling of the body coil to the one or more local coil(s) as much as possible. Coupling may cause detuning of the local coils, so an adjustment of the coil element to the receive amplifier is affected. Furthermore, noise from the body coil may also be coupled into the local coil. Both effects lead to a significant deterioration in a resulting image quality that may manifest itself in a worse signal-to-noise ratio and/or in altered spatial receiving characteristics of the local coil. For this reason, as a rule, either only the local coils or only the body coil are/is used nowadays for receiving magnetic resonance signals.
What are known as detune circuits are used in conventional body coils to suppress coupling of the body coil with local coils during the receiving interval. These have, for example, PIN diodes and deactivate the body coil during the receiving interval. The function of the detune circuit conventionally consists in disconnecting current paths in the antenna unit of the body coil and/or shifting an operating frequency of the antenna unit that conventionally corresponds to the Larmor frequency so optimally low coupling to the local coils occurs.